The invention relates to a line synchronizing circuit for a picture display device, comprising a control loop for controlling the frequency and/or the phase of a line oscillator, with means for applying an incoming line synchronizing signal and a reference signal generated by the oscillator to a phase discriminator for determining the phase difference between the applied signals and with a low-pass filter for smoothing the output signal of the phase discriminator and for applying the smoothed control signal obtained to the oscillator for controlling this oscillator, the circuit also comprising a coincidence detector for establishing that a phase difference between the incoming line synchronizing signal and the reference signal is less than a predetermined value and for switching elements of the control loop for bringing the loop into a first or a second state, respectively, in which second state the loop gain and the pull in rate, respectively, have a higher value than in the first state.
Such a synchronizing circuit is disclosed in the British Patent Specification 1,557,697. In this prior art circuit, elements of the control loop are changed over for obtaining, in the absence of coincidence, a large pull-in range and a rapid pull-in of the oscillator. For this purpose the time constant of the low-pass filter is changed to a lower value while the loop gain is given a higher value. As soon as the oscillator has the frequency and substantially the phase of the incoming line synchronizing pulses, this is to say as soon as coincidence is detected, the time constant of the filter is switched to the higher value thereof, in which state noise and interference have less influence on the oscillator, while the oscillator needs only to be readjusted to a very small degree. If coincidence is lost, for example because a change-over to a different television transmitter is effected, then an immediate switch to the lower value of the time constant is effected.
In the prior art synchronizing circuit coincidence may occur incidentally, without the oscillator having been really pulled-in, that is to say the frequency and the phase thereof are not near the target values to be achieved. Because the coincidence detector has detected coincidence, the time constant of the filter is switched to the higher value thereof and the loop gain become smaller, causing the pull-in procedure to proceed more slowly. This causes the duration of pull-in to be unnecessarily long, particularly in the case that much noise is initially received. This is a drawback, more specifically with automatic tuning systems.